Soil clay mineralogy at Rothamsted began in the early 1930s, and quickly focused on technique, swelling minerals, micas and sorption phenomena. By the mid-1940s interest had extended to the formation and spatial distribution of soil clays with the move of the Soil Survey of England and Wales to Rothamsted. These themes continued for the next 40 years. Considerable contributions were made to crystal chemistry and structure determinations, and the relationship of these to the sorption and desorption of water and organic molecules, and the behaviour of soil K. The pattern of soil clay mineralogy was determined for large parts of England and Wales, and absorbed into soil mapping. The work on X-ray diffraction appeared in the Mineralogical Society Monographs on this subject. These, and the Monograph on Clay Chemistry, which reached their final form in the 1980s, are the lasting monuments to soil clay mineralogical research at Rothamsted.

An informal overview is given of the development of the fundamental particle model, where, in a series of papers published from 1982-1987, researchers from the Macaulay Institute presented a radical model for the interpretation of the crystal structure, chemistry, and genesis of interstratified clays. The model reconciled electron microscopy and X-ray diffraction (XRD) data from a variety of sedimentary clay specimens, and proposed that these minerals were composed of nano-crystalline ‘fundamental particles’ whose adsorptive interfaces were responsible for the expanding smectite layers observed by XRD. The term ‘interparticle diffraction’ was used to describe this phenomenon. Experiments were reported which proved the model, by producing randomly interstratified illite-smectites from combinations of ordered illite-smectite and smectite clay dispersions. The model was extended to propose that fundamental particles were the primary units of crystallization, and that changes in the particle population were responsible for the commonly observed evolution in XRD character of these minerals with increasing depth of burial and temperature.

The kaolinized granites of St. Austell, England, are worked to produce a range of china clay products, for some of which the kaolin has to meet stringent particle shape and size specifications. Systematic petrographic study indicates that kaolin occurs in the form of two textural types: (i) finely crystalline kaolin (typically <5 µm in average diameter), which infills dissolution porosity of granitic feldspars, and (ii) coarsely crystalline vermiform aggregates (up to 100 µm or more in length), which are closely associated with expanded micas. The vermiform aggregates are characterized by an intergrowth of mica and kaolin crystals, which can be observed at scales of resolution offered by TEM. Textural and chemical evidence suggest that the expanded mica texture is probably the result of preferential precipitation of kaolin along mica cleavage planes and is not simply a process of chemical replacement.

Petrographic examination of kaolin slurries sampled at different points in a typical refinery circuit indicates that platy products with high aspect ratio are derived exclusively from raw materials rich in vermiform aggregates. The fine scale intergrowth of kaolin and mica within the aggregates results in circumstances where mica persists through to fine grained products. Furthermore, the absence of Fe or other chemical components in the kaolin structure suggests that any iron reported for the final products may be a consequence of the presence of Fe-bearing mica within a very fine grained intergrowth.